As an automatic two-wheeled vehicle, a frame structure in which a seat frame is welded to a back portion of a pivot frame is disclosed (for example, see Patent Document 1).
Patent Document 1 discloses a frame structure for a compact vehicle having a head pipe, a main pipe extending rearward and downward from the head pipe, a pair of left and right center upper pipes welded to a rear portion of a main pipe and extending rearward, center pipes extending downward from the center upper pipes, seat rails welded at upper portions of front ends thereof to rear ends of the center upper pipes and extending rearward, rear pipes extending downward from lower portions of the front ends of the seat rails, and a pair of left and right pivot frames connected to the lower portions of the rear pipes.
In this compact vehicle, the pivot frames are formed into a gusset shape, the rear pipes extending downward from the lower portions of the front ends of the seat rails are provided, and outer edges of the pivot frames and peripheral surfaces of the rear pipes are welded.
In this frame structure, a seat load acts on both of the rear pipes and the center upper pipes, and a vehicle body frame has sufficient rigidity with respect to the seat load by providing large spaces between the center upper pipes and the rear pipes. Therefore, the strength of welded portions between the rear pipes and the pivot frames is secured.